Ligand-Directed Metalation of a Gold Pyrazolate Cluster

Solid “[AuL]” (HL = 3-[pyrid-2-yl]-5-tertbutyl-1H-pyrazole) can be crystallized as cyclic [Au3(μ-L)3] and [Au4(μ-L)4] clusters from different solvents. The crystalline tetramer contains a square Au4 core with an HT:TH:TH:HT arrangement of ligand substituents, which preorganizes the cluster to chelate to additional metal ions via its pendant pyridyl groups. The addition of 0.5 equiv of AgBF4 to [AuL] yields [Ag2Au4(μ3-L)4][BF4]2, where two edges of the Au4 square are spanned by Ag+ ions via metallophilic Ag···Au contacts. Treatment of [AuL] with [Cu(NCMe)4]PF6 affords the metalloligand helicate [Cu2Au2(μ-L)4][PF6]2, via oxidation of the copper and partial fragmentation of the cluster.

C oinage metal pyrazolate salts adopt oligomeric structures, with trimeric and tetrameric molecular and 1D polymeric structure types being well-known in the solid state. 1−4 The [M 3 (μ-pz) 3 ] (M = Cu, Ag, or Au; Hpz = 1H-pyrazole, or a substituted derivative) cyclic trimer is the most common motif in these compounds. 4 These are essentially planar, notwithstanding any peripheral substituents, and often aggregate in the crystal through short M···M contacts. Such compounds can show an intense, temperature-dependent emission in the solid state, 5−10 from transitions within the intermolecular metallophilic orbitals. 11−13 Similarly, hybrid or soft materials based on [M 3 (μ-pz) 3 ] centers can show switchable emission mediated by reversible supramolecular aggregation processes. 14−18 Triangular [M 3 (μ-pz) 3 ] centers with appropriate substituents can be π-acid hosts for aromatic guest species, 1,4,19,20 and D 3h -symmetric synthons in crystalline frameworks 21,22 and in 2D coordination nanosheets. 23 Sterically hindered 3-(pyrid-2-yl)-5-tertbutyl-1H-pyrazole (HL) supports a number of novel metal−organic molecular architectures. 24−28 For example, the silver chemistry of HL afforded a rare example of metallophilic isomerism in two polymorphs of [Ag 3 (μ-Br)(μ-L) 2 ], and the largest known homoleptic coinage metal pyrazolate cluster [Ag 10 (μ-L) 8 ] 2+ . 28 We were therefore intrigued to study complexes of HL with other coinage metals. We report here the isolation of two clusters [Au n (μ-L) n ] (n = 3 or 4) and their further reaction with other metal sources to form mixed-metal compounds. This has resulted in a rare postsynthetic metalation of a preformed gold(I) cluster with Ag(I), without inducing any further structural rearrangement. 29−36 Dropwise addition of NBu 4 OH solution to an equimolar suspension of [AuCl(tht)] (tht = tetrahydrothiophene) and HL 37 in methanol affords a clear, pale yellow solution. Storage of the filtered solution at 255 K for 3 days yields an off-white microcrystalline precipitate analyzed as [AuL] (1). The synthesis is unpredictable and sometimes yields colloidal gold rather than the desired complex 1. Analogous reactions using different solvents and bases also suffered from this problem while giving lower yields of 1 when they worked as desired. Since most gold pyrazolates precipitate cleanly when synthesized from polar solvents, the sensitivity of this reaction might reflect the chelating N-donors in [L] − , which could be incompatible with the preferred linear coordination of gold(I) in the reaction mixture. Be that as it may, once isolated, 1 is stable under ambient conditions and soluble in weakly polar solvents.
Recrystallization of 1 from chlorinated solvents affords mixtures of colorless crystals and an amorphous material. There are no close intramolecular steric contacts between the pyridyl or tertbutyl substituents that might influence the ligand arrangement in 1b·xEt 2 O. However, the same isomer is a major component in solutions of 1, as described below. In contrast, another [Au 4 (μ-pz) 4 ] complex with an unsymmetric pattern of bulky pyrazole substituents adopts the more symmetrical HT:HT:HT:HT isomer in the solid state. 38 The Au···Au distances in 1a are all 3.3529(7) Å, while in 1b they range between 3.1661(8) and 3.3053(8) Å. These are typical dimensions for these classes of compounds and imply only weak interactions between the metal ions in each molecule. There are no close intermolecular Au···Au contacts in the lattices of 1a and 1b·xEt 2 O, which presumably reflects the steric bulk of their tert-butyl groups ( Figures S3 and S4).
Bulk samples of 1 are a mixture of at least two phases determined by powder diffraction, including 1a and a phase related to 1b·xEt 2 O. Unfortunately, we have been unable to purify bulk samples of 1a and 1b for separate characterization.
Some other coinage metal pyrazolate complexes have also been crystallized in more than one aggregation state, 39−43 which can exist in concentration-dependent equilibria in solution. 43−45 However, the ESMS spectrum of 1 shows a strong molecular ion for [1b + H] + (m/z = 1589.3473) but no peak assignable to 1a. Hence, 1a should be a minor component in solutions of 1, even though it can be crystallized under some conditions. Other coinage metal pyrazolates with tert-butyl ligand substituents also prefer tetranuclear over trinuclear structures, probably on steric grounds. 38 Figures S7 and S8). 48 For consistency with the mass spectrum, we assign these to three isomers of tetranuclear [Au 4 (μ-L) 4 ], respectively with HT:HT:HT:HT, HT:TH:TH:HT (i.e., 1b), and HT:TH:HT:HT pyrazole substituent patterns (Chart S2).
The disposition of the ligands in 1b places its pyridyl substituents adjacent to each other across two edges of the Au 4 square (Figure 1). That could preorganize them to chelate to additional metal ions. 29,32,35,49−54 We therefore explored reactions of preformed 1 with additional equivalents of other coinage metal precursors. No reaction was observed between 1 and [Au(tht) 2 ]PF 6 , which gave unchanged 1 as the only isolable product. However, treatment of 1 with 0.5 equiv AgBF 4 per the "[AuL]" formula unit in thf affords a new offwhite product, [Ag 2 Au 4 (μ 3 -L) 4 ][BF 4 ] 2 (2). 39 Compound 2 is soluble in MeCN and MeNO 2 , but it does not form single crystals from those solvents. However, single crystals of formula 2·yC 2 H 4 Cl 2 , y ≈ 3.6) were grown by slow evaporation of a solution of 2 in 1,2-dichloroethane, in which it is only sparingly soluble.
The structure of 2 contains a square [Au 4 (μ-L) 4 ] moiety, whose geometry and metric parameters are almost identical to those of 1b within experimental error. However, two adjacent edges of the square assembly are now spanned by silver ions (Figure 2). Each Ag(I) ion is coordinated by the pyridyl Ndonor atom from two [L] − ligands, as predicted, with an additional long contact to a partially occupied, disordered solvent molecule. The N−Ag−N angle is significantly bent at 135.8(4)°, which orients each silver atom toward the midpoint of a Au···Au vector. The Ag···Au distances of 2.9344(12)− 2.9346(10) Å lie within the midrange for metallophilic (d 10 − d 10 ) bonding interactions between those two metals. 55 The

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Au···Au distances span a narrower range than in 1b, at 3.1645(6)−3.1878(7) Å ( Figure S10) 3 CN showed a single species with two equally populated L environments, which is the symmetry expected for both 1b and 2 ( Figure S13). Silver complexes of heterocyclic ligands are often labile in solution. 28,43−45,56−60 However, the pyridyl H 3 resonances in 2 lie 0.5−0.6 ppm upfield compared to 1b, implying that silver coordination to those residues is retained in the sample. Consistent with that, the ESMS spectrum of 2 contains a strong peak assigned to [AgAu 4 L 4 ] + (m/z = 1697.2449), as well as a peak from demetalated 1b ( Figure S14).
The absorption spectra of 1 and 2 in MeCN at 298 K are similar to HL, 37 being featureless in the visible region but with an envelope of intense pyridyl π−π* transitions around λ max = 258 nm. Excitation of 1 at 270 nm yields an intense structured emission with maxima at λ max = 337 and 385 nm, which is probably ligand-centered, 37,65 and a weaker emission at λ max = 654 nm (Figures 4 and S18). This resembles the orange emission shown by solutions of [Au 4 (pz tBu2 ) 4 ] (Hpz tBu2 = 3,5di(tertbutyl)-1H-pyrazole), 46 which arises from metal-to-metal charge transfer (MMCT) transitions within the metallophilic Au···Au orbital manifold. 12,13,33,47 The emission spectrum of 2 (λ ex = 250 nm) resembles that of 1, but the main visible emission is slightly red-shifted at λ max = 667 nm. The spectrum also has weak additional features in the visible region which may be vibrational structure on the emission bands, reflecting the greater conformational rigidity of 2. 35 A number of [{Au 3 (μ-L) 3 } 2 Ag] + clusters have been reported, where [L] − is a substituted pyrazolate or a C,Ndonor 1,2-bridging ligand. These contain a silver(I) ion sandwiched by two cyclic trigold(I) metalloligands, through unsupported Au···Ag metallophilic interactions. 30−34 The sandwich assemblies can show enhanced room-temperature phosphorescence compared to the trigold precursors, 33,34 which has been exploited in emissive soft materials 14−16 and silver ion sensors 17,18 containing embedded trigold clusters. Higher nuclearity [Au n (μ-L) n ] (n > 3) complexes are less common, but our synthesis of 2 shows that they can also be decorated with silver ions, using pendant ligand substituents to direct the metalation. 35,36 This is a promising strategy toward heterometallic pyrazolate clusters of gold and other coinage metals. 32